Lecture%2006%20Mechanical%20Properties

Lecture%2006%20Mechanical%20Properties - MECH 340...

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Unformatted text preview: MECH 340 Engineering Material Mechanical Properties Area, A F t F t Objectives • Introduce the basic concepts associated with mechanical properties of materials. • Evaluate factors that affect the mechanical properties of materials. • Review some of the basic testing procedures that engineers use to evaluate many of these properties. Terminology • Stress- Force or load per unit area of cross-section over which the force or load is acting. • Strain- Elongation change in dimension per unit length. • Young’s modulus- The slope of the linear part of the stress-strain curve in the elastic region, same as modulus of elasticity. • Shear modulus (G)- The slope of the linear part of the shear stress-shear strain curve. • Viscosity ( )- Measure of resistance to ¡ow, de¢ned as the ratio of shear stress to shear strain rate (units Poise or Pa-s). • Thixotropic behavior- Materials that show shear thinning and also an apparent viscosity that at a constant rate of shear decreases with time. Common States of Stress • Simple tension: cable o σ = F A A o = cross sectional Area (when unloaded) F F σ σ • Simple shear: drive shaft o τ = F s A τ Note: τ = M / A c R here. M M A o 2R F s A c Common States of Stress Canyon Bridge, Los Alamos, NM • Simple compression: A o Balanced Rock, Arches National Park o σ = F A Note: compressive structure member ( σ < 0 here). Common States of Stress • Bi-axial tension: Pressurized tank σ z > 0 σ θ > 0 • Hydrostatic compression: Fish under water σ < 0 h Response to Applied Load The response of engineering materials to an applied stress can take many form depending on the structure and properties of the stressed material pure elastic elastic plastic visco-elastic rubbery viscous creep relaxation Newtonian and non- Newtonian Liquids Shear stress strain rate relationships for Newtonian and non-Newtonian materials The Tensile Test Load - The force applied to a material during testing. Strain gage or Extensometer- A device used for measuring change in length and hence strain. Glass temperature (Tg )- A temperature below which an otherwise ductile material behaves as if it is brittle. Engineering stress - The applied load, or force, divided by the original cross- sectional area of the material. Engineering strain - The amount that a material deforms per unit length in a tensile test. Tensile Test The stress-strain curve for an aluminum alloy Example An aluminum rod is to withstand an applied force of 45,000 pounds. To assure a sufFcient safety, the maximum allowable stress on the rod is limited to 25,000 psi. sufFcient safety, the maximum allowable stress on the rod is limited to 25,000 psi....
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This note was uploaded on 02/07/2011 for the course MECH 340 taught by Professor Marwandarwiche during the Summer '07 term at American University of Beirut.

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Lecture%2006%20Mechanical%20Properties - MECH 340...

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